This invention relates to the recovery of maleic anhydride, and particularly to the recovery of maleic anhydride from gaseous mixtures containing maleic anhydride in the vapor phase using an organic solvent.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
Maleic anhydride can be prepared commercially by the selective partial oxidation of suitable starting materials, such as benzene, butene or butane, with an oxygen-containing gas using an appropriate catalyst. Both fluidized bed reactors and fixed-tube heat-exchanger type reactors can be used for such partial conversions, and the details of the operation of such reactors are well known to those skilled in the art. Generally, the reaction to convert the hydrocarbons to maleic anhydride requires only passing the hydrocarbon admixed with a free-oxygen containing gas, such as air or oxygen-enriched air, through a vanadium-type catalyst at elevated temperature. The hydrocarbons are passed through the catalyst at a concentration of about 1.5 to about 10 volume percent hydrocarbon at a space velocity of about 100 to 4,000 cc/cc/hour at temperatures between about 350.degree. C. and 600.degree. C. In such processes, a gaseous reaction mixture is produced which contains from about 0.5 to about 2% by volume of the anhydride together with byproducts such as carbon monoxide, carbon dioxide, water vapor, inert gases, and the like.
The prior art also discloses a number of methods of recovering the maleic anhydride from the gaseous reaction mixture leaving the reactor. As an example, the maleic anhydride can be recovered by direct condensation and this method is widely used in commercial operations. On the other hand, the prior art also discloses that the maleic anhydride can be absorbed in a suitable solvent with subsequent separation and purification of the anhydride. As an example, British patent specification No. 763,339 discloses a process for scrubbing maleic anhydride from gaseous reaction mixtures using dibutyl phthalate as the solvent, and thereafter stripping the maleic anhydride from the solvent in two steps, each under critically controlled conditions, to recover the maleic anhydride. U.S. Pat. No. 2,574,644 describes a process for the recovery of both phthalic anhydride and maleic anhydride using dibutyl phthalate as the solvent, and thereafter separating each anhydride by modifying the operating conditions in the recovery steps. British patent specification No. 727,828 also describes a process for the recovery of phthalic anhydride and maleic anhydride in a gaseous reaction mixture wherein the reaction mixture is passed in contact with a dibutyl phthalate solvent for simultaneous absorption of both anhydrides, and stripping the absorbed anhydrides from the solvent. U.S. Pat. No. 3,891,680 discloses a process for the recovery of maleic anhydride from gaseous reaction mixtures by contacting the gaseous reaction mixture with a dialkyl phthalate ester having 4 to 8 carbon atoms in each alkyl group and a total of 10 to 14 carbon atoms in both alkyl groups.
Although maleic anhydride can be recovered from gaseous reaction mixtures using these and other processes in the prior art, all of these processes contain one or more disadvantages. As an example, in the conventional method of recovering maleic anhydride from gas mixtures by condensation, only about half of the maleic anhydride is recovered, and the gaseous reaction mixture must be processed further by other methods to recover the remainder of the maleic anhydride. In another method, the gaseous reaction mixture is scrubbed with water and the resulting aqueous mixture of maleic anhydride is dehydrated by azeotropic distillation with xylene. This recovery process has the drawback of converting a fair amount of maleic anhydride into fumaric acid, which in turn, causes yield losses and plugging problems in the maleic anhydride recovery sections of the plant.
The prior art processes using phthalate esters as the solvent teach that the maleic anhydride must be stripped from the solvent in two steps, each under critically controlled conditions, to separate the maleic anhydride from the solvent. Some processes relate to the recovery of a mixture of maleic anhydride and phthalic anhydride from gaseous reaction mixtures, but in today's commercial market these anhydrides are seldom produced as a mixture, but are generally produced in relatively high purity in plants designed to produce either maleic anhydride or phthalic anhydride. Other processes contemplate the use of solvents which are expensive or thermally unstable or unsuitable for other reasons for use in a commercial operation.
These and other disadvantages of the prior art are overcome by the process of the present invention. By the process of the present invention, greater than 98% of the maleic anhydride can be recovered from a gaseous reaction mixture using a solvent that is thermally stable, and which does not require multiple stripping steps under critically controlled conditions to separate the maleic anhydride from the solvent. Furthermore, although the process is suitable for the recovery of maleic anhydride obtained by the partial oxidation of benzene, it is particularly applicable to the recovery of maleic anhydride in a gaseous reaction mixture obtained by the partial oxidation of an olefin, such as butene, or of a saturated aliphatic hydrocarbon, such as butane, since it is known to those skilled in the art that the gaseous reaction mixture from the partial oxidation of an aliphatic hydrocarbon contains a greater amount of water vapor than does the gaseous reaction mixture from the partial oxidation of benzene. By the process of the present invention, the maleic anhydride is almost quantitatively removed from the gaseous reaction mixture using the solvent of the present invention, while at the same time maleic anhydride can be recovered from the solvent in a one-step stripping operation with an overall process efficiency of greater than 98 percent.